This invention relates to novel compositions containing heat transfer fluids and solvents, and more particularly, it relates to novel chlorofluorocarbon compositions useful in absorption heating and cooling apparatus.
Machines operating on the principle involving extraction, through absorption, of low-level heat energy derived, for instance, from outside air for a heat pump or from a chilled space for a cold-producing unit, and on the return of this energy to the premises to be heated in the first case, or to the surrounding space in the second case, make use of a readily vaporizable material (the heat transfer working fluid) and a low-volatility material which is preferably a good solvent for the former. In a heat pump, the solution enriched in heat transfer fluid after absorption returns to the boiler where the two fluids are once again separated by heating. The heat transfer fluid is vaporized and conveyed to the pump circuit where it first undergoes condensation at an elevated temperature and then pressure reduction and subsequent evaporation before returning to the absorber.
The selected heat transfer working fluid/solvent pairs must meet a number of criteria related to the practical operation of such machines:
(a) The boiling points under normal pressure must be separated by at least 150.degree. to 200.degree. C. to avoid the use of a reflux device for the solvent;
(b) The boiling point of the heat transfer fluid must be neither too low, to avoid the use of high service pressures, nor too high, to satisfy (a);
(c) The proportions of the two materials employed must be completely miscible over a wide temperature range;
(d) The solvent must remain liquid at low temperature and have a solidification point which is as low as possible to maintain an acceptable viscosity at temperatures of the order of -20.degree. to 0.degree. C.;
(e) The mixture must have excellent thermal and chemical stability in the temperature range involved in the thermodynamic cycle, that is, at a temperature region ranging, in the case of heat pumps in particular, from -10.degree. C. for the cold source (absorber) to 150.degree.-180.degree. C. for the boiler; and
(f) Finally, the products chosen must not be toxic in any way.
For safety reasons and also on account of their good thermal and chemical stability, the majority of heat transfer fluids are selected from fluorinated hydrocarbons and, especially, from chlorofluorinated hydrocarbons containing one or two carbon atoms. These best meet the volatility and solubility criteria set forth above. The most widely employed compounds are dichlorodifluoromethane (R-12), dichlorofluoromethane (R-21), chlorodifluoromethane (R-22), chlorofluoromethane (R-31), 1,2-dichloro-1,1,2,2-tetrafluoroethane (R-114), 1,1-dichloro-1,2,2,2-tetrafluoroethane (R-114a), chloropentafluoroethane (R-115), 1,1-dichloro-2,2,2-trifluoroethane (R-123), 1,2-dichloro-1,2,2-trifluoroethane (R-123a), 1,1-dichloro-1,2,2-trifluoroethane (R-123b), 1-chloro-1,2,2,2-tetrafluoroethane (R-124), 1-chloro-1,1,2,2-tetrafluoroethane (R-124a), 1-chloro-1,2,2-trifluoroethane (R-133), 1-chloro-2,2,2-trifluoroethane (R-133a), 1-chloro-1,1,2-trifluoroethane (R-133b), and 1-chloro-1,1-difluoroethane (R-142b).
When the fluorinated hydrocarbon contains one or more hydrogen atoms, the best solvents are those capable of forming intermolecular combinations via a hydrogen bond and are selected from the categories of carbonyl derivatives such as esters, ketones, amides, and lactams; alcohols; or polyethylene glycol ethers. Among the most widely used solvents, the dimethyl ethers of tri or tetraethylene glycol are shown in USSR Patent No. 150,113, Japanese Application No. 79-152,257, and German Application No. 3,202,377; N-methylpyrrolidone is shown in USSR Patent No. 643,524 and Japanese Applications Nos. 79-145,774 and 82-132,545; N,N-dimethylformamide is shown in Japanese Application No. 82-121,759; and dibutyl phthalate is shown in USSR Patent No. 346,326. Imidazolines and phosphoramides in Japanese Application No. 82-132,544 and German Application No. 2,944,189, and tetrahydrofurfuryl alcohol derivatives in U.S. Pat. No. 4,251,382 have also been recommended as solvents.
In absorption machine boilers, solutions enriched in the working fluid are heated to elevated temperatures. The walls of the boiler are generally metallic materials based on iron, aluminum or copper which, under the effect of the temperature, may constitute a destabilizing element for the heat transfer fluid/ solvent pair. It is well known, in fact, that a chlorofluorinated hydrocarbon of the type C.sub.n H.sub.2n+2 -x-yF.sub.x Cl.sub.y, heated in the presence of one of the aforementioned metals and a hydrogen-donor compound, undergoes a quantitative conversion with progressive substitution of the chlorine atoms by hydrogen atoms from the solvent. A greater or lesser proportion of ethylene derivatives is also formed when n is two or greater, depending on the reactivity of the donor and the structure of the fluorinated hydrocarbon. Thus, in the case of 1-chloro-2,2,2-trifluoroethane (R-133a), these conversions take place according to the following reaction scheme: EQU CF.sub.3 --CH.sub.2 Cl+Fe.fwdarw.CF.sub.3 --CH.sub.2 *+FeCl* EQU CF.sub.3 --CH.sub.2 *+R--H (solvent) ).fwdarw.CF.sub.3 --CH.sub.3 +R* EQU CF.sub.3 --CH.sub.2 *+FeCl*.fwdarw.CF.sub.2 =CH.sub.2 +FeClF
In the case of 1-chloro-1,2,2,2-tetrafluoroethane (R-124), heated in the presence of N-methylpyrrolidone and iron, this produces 1,1,1,2-tetrafluoroethane (R-134a) and a mixture of 1,1-difluoroethylene and trifluoroethylene.
This reaction is accompanied by the formation of tars at the expense of the solvent and by considerable corrosion due to the action of the resulting metal halides. The alteration in the nature of the heat transfer fluid/solvent pair, the corrosive effects, and particularly the formation of non-condensable gases produce deterioration in the heat pump operating parameters and can very rapidly cause it to break down.
Various suggestions have been made with a view to overcoming this disadvantage and ensuring a minimum reliability of the order of 10,000 hours. The simplest of these involves stabilizing the mixture of chlorofluorinated heat transfer fluid and solvent by introducing an additive in a proportion which does not significantly modify the physico-chemical and thermodynamic properties of the heat transfer fluid/solvent pair. Unfortunately, these additives are of only limited effectiveness with time, and their action can be summarized chiefly as an extension of the inhibition period.
Another procedure involves replacing the chlorofluorinated hydrocarbon by a polyfluorinated alcohol such as trifluoroethanol, as shown in Japanese Applications Nos. 80-16315, 81-88485, 82-132545, and 83-98137. However, the relatively high boiling points of these alcohols (73.5.degree. C. in the case of trifluoroethanol) cause a part of the apparatus (the evaporator) to operate at a pressure which is significantly below normal pressure.
Finally, the nature of the solvent can be modified and it can be chosen from a group of products which do not give rise to the foregoing reactions which take place in the presence of metals and chlorofluorinated hydrocarbons. From this point of view, structural identity has been exploited in French Patent No. 2,075,236 and U.S. Pat. No. 4,003,215, which employ pairs of chlorofluorinated hydrocarbons as solvent and working fluid. However, in the case of the pairs R-12/R-11 and R-12/R-113, mentioned in the Examples, the small boiling point differences (of the order of 40 to 80.degree. C.) make it necessary to use complicated techniques to carry out the solvent/heat transfer fluid separation at the boiler exit.